1. Field of the Invention
The invention relates to medical devices. Specifically, the invention relates to a porous metal expansion bolt for use in surgical procedures and the fixation and biologic integration of various tissues.
2. Description of the Related Art
Connective tissue fixation devices for securing tendon, ligament and bone to other connective tissues are typically fabricated from medically approved metallic or biodegradable materials. Although generally successful in achieving the goal of temporary mechanical fixation of graft tissue to target tissue, currently available connective tissue fixation devices have a number of well-known disadvantages.
A universal limitation to all currently described soft-tissue fixation devices is that ultimate biologic integration of tendon, ligament, and bone through in-growth healing occurs solely on the biologic side of the interface. No soft-tissue integration has ever been shown to be sustained directly to the implant, which, by all current designs, must occupy a substantial portion of the area available for graft-target tissue healing. This lack of in-growth area limits initial and ultimate strength of construct. In addition, the ability of the connective tissue to respond to future damage and functional remodeling is limited when fixation is achieved mechanically.
Biodegradable (or bio-absorbable) fixation systems have numerous well documented problems. These implants are brittle and may fail upon initial implantation, they may degrade rapidly and unpredictably with loss of fixation strength, and they may produce local and systemic host reactions leading to the rejection of the implant, catastrophic failure, and massive bone loss. An additional disadvantage of a current interference screw design is difficulty of screw removal in cases that require such hardware removal. In the case of bio-absorbable implants, frequently structural competency is lost and the fixation device breaks deep with in the host tissue; in the case of metallic implants, soft-tissue damage can occur when the threads re-engage the tenuous collagen structures. Interference screw tissue implants commonly may include a threaded shank joined to an enlarged head having a transverse slot, hexagonal, or star-shaped driver for advancing the screw into the bone. The enlarged heads or prominent variable pitch diameters on such screws may protrude from the implantation site resulting in chronic irritation, inflammation of surrounding body tissue, and early or late reconstruction failure.
When placed, as is common, in proximity to repetitively moveable joints, such medical screw fixation devices may, in certain instances, cause abrading of ligaments, tendons, nerves, arteries, veins, bone, cartilage, and skin during the normal physiologic motion of the joint. Fixation implants may back out after insertion, protruding into surrounding tissue and causing discomfort and morbidity.
Furthermore, due to a phenomenon known as “stress shielding,” fixation implants which do not closely match the elastic modulus (flexibility) of the surrounding host tissue may shield this connective tissue (bone, tendon, ligament) from beneficial stresses that enhance healing. It has been shown that moderate periodic stress on bone tissue, such as the stress produced by exercise, helps prevent decalcification and loss of the bone. Under these physiologic conditions, the stress shielding which results from the long-term use of implant-host modulus mismatch fixation devices can lead to osteoporosis, tissue weakness, and catastrophic failure.